1. Field of the Invention
The present invention relates to a retardation film and to a method of producing the same. To be specific, the present invention relates to a retardation film which hardly causes shift or unevenness in retardation values, which has excellent mechanical strength, and which can be applied to a large liquid crystal display apparatus, and to a method of producing the retardation film. Further, the present invention relates to an optical film, a liquid crystal panel, and a liquid crystal display apparatus all using the retardation film.
2. Description of the Related Art
A liquid crystal display apparatus is generally provided with a polarizer on both sides of a liquid crystal cell. Further, the liquid crystal display apparatus is conventionally provided with a retardation film between the liquid crystal cell and the polarizer for optical compensation of retardation due to birefringence of the liquid crystal cell in a front direction and an oblique direction. In order to improve display properties of the liquid crystal display apparatus in an oblique direction, it is very important to determine how retardation values of the retardation film change in an oblique direction with varying angles.
There is disclosed a retardation film in which retardation values are substantially constant regardless of angles (JP 02-160204 A, for example). The retardation film has a refractive index profile of nx>nz>ny, when nx, ny, and nz represent refractive indices in a slow axis direction, fast axis direction, and thickness direction of the film, respectively. There is also disclosed a liquid crystal display apparatus of IPS mode or VA mode with improved viewing angle properties in an oblique direction by using a polarizing plate employing the retardation film or by using the retardation film (JP 11-305217 A, JP 2000-39610 A, and JP 04-305602 A, for example)
There is disclosed a method of producing a retardation film having a refractive index profile of nx>nz>ny involving: bonding a shrinkable film on one side or both sides of a polymer film for formation of a laminate; and subjecting the laminate to heat stretching treatment (JP 05-157911 A, for example). In the production method, a refractive index profile of the polymer film must be changed greatly before and after the stretching. Thus, the polymer film to be preferably used easily causes retardation with a low stretch ratio, and examples thereof generally used include aromatic polymer films of a polycarbonate-based resin, a polyarylate-based resin, a polysulfone-based resin, and the like. However, the aromatic polymer film has a large photoelastic coefficient and has an easily varying retardation by stress. Thus, the aromatic polymer film has a problem of degrading display properties as described below. In a case where an aromatic polymer film attached between a liquid crystal cell and a polarizer is exposed to high temperatures, retardation values may depart from designed values due to shrinkage stress of the polarizer. In a case where an aromatic polymer film is used for a liquid crystal display apparatus, uneven stress generated by heat of backlight may cause unevenness in retardation values. Meanwhile, an aliphatic resin film such as a norbornene-based resin film has a small photoelastic coefficient. However, the aliphatic resin film hardly causes retardation, and desired retardation values cannot be obtained through stretching at a low stretch ratio as in the aromatic polymer film, or even at a high stretch ratio. The stretching at a high stretch ratio causes a problem of breaking of the film.